Sensors are transducers or converters that measure a physical quantity and convert it into a signal which can be read. Typically, that reading is by an electronic instrument which converts the signal to a measurement based upon the sensitivity of the sensor, its calibration data, and other corrections. Included within the many types of sensors are those relating to sound, acoustics, vibration, chemicals, humidity, pressure, fluid flow, position, displacement, rotation, force, level, temperature, proximity, and acceleration. For each type of sensor, different sensing mechanisms exist which may for example be targeted to different dynamic ranges, speed, accuracy, etc. Amongst these, capacitive sensing constitutes a very important means of monitoring many environmental conditions in an effective and reliable manner as well as having applications in areas including, but not limited to, gas sensing, thickness measurements, haptic interfaces, health and fitness sensing, appliances monitoring, consumer electronics sensing, industrial sensing, building automation, wireless sensing, heating, ventilation, and air conditioning system monitoring, and displacement measurements. Accordingly, capacitive sensors have major applications in the consumer, industrial, automotive and medical fields.
It is very desirable to miniaturize and integrate such capacitive sensing systems in order to meet the requirements of existing markets and penetrate new markets and reduce fabrication costs through batch processing. Sustainable protection from oxidation, high temperatures (<350° C.) and corrosion are also especially critical for operation in harsh environments. In many instances the integration of capacitive based sensors directly with their associated electronics is important in attaining packaging dimensions and costs that are compatible with very low-cost high volume markets, such as consumer electronics for example. In addition, this integration can bring forward enhanced performance through the optimal interconnection with signal processing electronics or added functionality through the inclusion of many sensing devices, with marginal cost of system footprint.
Microelectromechanical systems (MEMS) have become a successful sensing and actuation technology. Because of their extensive optical, electrical to mechanical (and vice-versa) functionalities, MEMS devices and transducers (that convert analog environmental quantities to electrical signals) are suited to applications in many different fields of science and engineering. However, because of this vast range of functionality, MEMS fabrication processes, unlike the microelectronics industry, are difficult to gear towards general applications. As a result, most processes historically have been aimed at the fabrication of a few specific device types, and usually performance of the devices is hindered by process variability. As MEMS devices and transducers are typically sensing weak analog signals, for example pressure, acceleration, vibration, magnetic or electric fields, with capacitive based elements, there is considerable benefit in being able to tightly integrate analog front-end electronics to buffer, amplify and process these weak electronic signals and either facilitate their direct processing, such as with RF signals, or their digitization for sensing and measurements applications.
Silicon CMOS electronics has become the predominant technology in analog and digital integrated circuits. This is essentially because of the unparalleled benefits available from CMOS in the areas of circuit size, operating speed, energy efficiency and manufacturing costs which continue to improve from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In respect of MEMS systems, CMOS is particularly suited as digital and analog circuits can be designed in CMOS technologies with very low power consumption. This is due, on the digital side, to the fact that CMOS digital gates dissipate power predominantly during operation and have very low static power consumption. This power consumption arising from the charging and discharging of various load capacitances within the CMOS gates, mostly gate and wire capacitance, but also transistor drain and transistor source capacitances, whenever they are switched. On the analog side, CMOS processes also offers power savings by offering viable operation with sub-1V power supplies and with μA-scale bias currents and below sub-μA sleep currents.
However, combining CMOS and MEMS technologies has been especially challenging because some MEMS process steps—such as the use of special materials, the need for high temperature processing steps, the danger of contamination due to the MEMS wet etching processes etc.—are incompatible with the requirements of CMOS technology. Thus, strong attention has to be paid to avoid cross contaminations between both process families. Accordingly, today MEMS processes exist that are discrete and standalone, such as Robert Bosch's (U.S. Pat. No. 5,937,275 “Method of Producing Acceleration Sensors”, MEMSCAP's “Multi-User MEMS Processes” (MUMPs® including PolyMUMPs™, a three-layer polysilicon surface micromachining process: MetalMUMPs™, an electroplated nickel process; and SOIMUMPs™, a silicon-on-insulator micromachining process), and Sandia's Ultra-planar Multi-level MEMS Technology 5 (SUMMiT V™ Fabrication Process which is a five-layer polycrystalline silicon surface micromachining process with one ground plane/electrical interconnect layer and four mechanical layers).
Other processes have been developed to allow MEMS to be fabricated before the CMOS electronics, such as Analog Devices' MOD-MEMS (monolithically integrate thick (5-10 um) multilayer polysilicon MEMS structures with sub-micron CMOS), and Sandia's iMEMS. Finally, processes have been developed to provide MEMS after CMOS fabrication such as Sandia's micromechanics-last MEMS, Berkeley Sensor & Actuator Center (BSAC), and IMEC silicon-germanium processes. Additionally DALSA Semiconductor have a highly publicized “low temperature” micro-machining with silicon dioxide process, see L. Ouellet et al (U.S. Pat. No. 7,160,752 “Fabrication of Advanced Silicon-Based MEMS Devices”, Issued Jan. 9, 2007) wherein low stress structures were fabricated at temperatures between 520° C. and 570° C., being just below the temperature of eutectic formation in aluminum-silicon-copper interconnections.
However, the mechanical properties of silicon do not make it the most suitable structural material for most MEMS. Recently, silicon carbide (SiC) has generated much interest as a MEMS structural material because of its distinctive and improved properties including for example higher acoustic velocity, high fracture strength, desirable tribological properties, ability to sustain higher temperatures, and resistance to corrosive and erosive materials. To date difficulties with SiC processing have made its use non-trivial as it is non-conductive and difficult to deposit and dope at temperatures that do not damage CMOS electronics (also referred to as being CMOS-compatible temperatures). Stress control is also difficult because of the high intrinsic stresses that can develop in such a material and because if its intrinsic inertness, selective etching of SiC is difficult. As most materials are etched at a faster rate than SiC, issues arise when masking SiC for patterning and ensuring a reliable etch-stop. Whether it is for doping or for deposition, SiC processing generally has been carried-out at high temperatures and as such prior art SiC MEMS processes have not lent themselves well to CMOS integration nor to use within capacitive sensing devices that exploit materials whose properties change under exposure to the measurand, for example water vapor (humidity), methane, carbon monoxide, and other chemicals, gases, and fluids. Such materials typically have even lower maximum processing temperatures than silicon CMOS electronics. Further as most MEMS and capacitive applications require electrical signal processing, integration of MEMS to transistor-able processes, such as CMOS, is paramount.
Within the prior art, a low temperature SiC processing technique has been described by the inventors in U.S. Pat. No. 8,071,411 entitled “Low Temperature Ceramic Microelectromechanical Structures,” U.S. Patent Applications 2011/0,027,930 entitled “Low Temperature Wafer Level Processing for MEMS Devices” and 2011/0,111,545 entitled “Low Temperature Ceramic Microelectromechanical Structures” and research publications including “Low-Stress, CMOS-Compatible Silicon Carbide Surface Micromachining Technology Part-I: Process Development and Characterization” (J. MEM Systems, Vol. 20, pp 720-729) and “Low-Stress, CMOS-Compatible Silicon Carbide Surface Micromachining Technology Part-II: Beam Resonators for MEMS Above-IC” (J. MEM Systems, Vol. 20, pp 730-744). The process outlined provides SiC structures with metallization formed on the upper surface of the SiC, the lower surface of the SiC, and optionally both surfaces. Typical structures within the work of the inventors in these initial publications and patents include capacitors, switches, and resonators wherein the structures included anchoring in one or more locations and electrostatic actuation.
However, as discussed supra in respect of MEMS sensors and capacitive sensors critical considerations for users include accuracy, repeatability, long-term stability, ability to recover from condensation and/or saturation, ease of calibration, resistance to chemical and physical contaminants, size, packaging, and cost effectiveness. Accordingly, it would be beneficial to exploit MEMS processes that allow for manufacturability and integration of SiC with silicon CMOS electronics, to effectively harness the benefits of SiC, and for these MEMS processes to allow integration of reference structures, electrical heaters, and electrical interconnections within the MEMS elements. It would be further beneficial for the capacitive sensors and MEMS elements to be implemented directly atop silicon CMOS electronics (i.e. above integrated circuits, or above-IC). Moreover, it would be beneficial to allow for the protection of a sensing layer by a SiC protective layer. Accordingly, the invention provides for a SiC-based MEMS process based capacitive sensing methodology. The invention providing further a route to very low-cost and high manufacturability process implementations with protection of the sensing material via SiC and above-CMOS integration capability.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.